Our research program encompasses studies of allotypes and idiotypes of mouse immunoglobulins and their role as genetic markers. These markers are used in the investigation of the genetic organization of the Ig gene complexes and in the regulation of their expression in the immune response. Idiotypic determinants T15 and C3, expressed on anti-phosphorylcholine antibodies and U61, expressed on anti-levan antibodies, were used as Igh-V region structural markers in inbred and wild mice. T15 and C3 behave like allelic markers in inbred mice but in wild mice were frequently found to have recombined. Wild mice of species that were distantly related to Mus musculus frequently exhibit none of the known allotypes but sometimes express T15 and U61 but not C3. More closely related wild mouse species exhibited the same allotypic markers as inbred mice and also expressed T15, C3 and U61 markers. However, the associations of allotypes and idiotypes seen in inbred mice were not observed in wild mice. These findings suggest that recombination may have occurred frequently in evolution or possibly that idiotypic and allotypic expression are regulated by other genes.